1. Field of the Invention
This invention generally relates to the detection of a signal and more specifically to a method and apparatus for estimating and utilizing phase discontinuities to better detect the presence of a corrupted signal in background noise.
2. Description of the Prior Art
In many applications involving energy transmitted through some medium to be reflected off an object, the signals representing the received reflected energy are characterized by phase discontinuities and background noises. In a sonar system, a transmitted acoustic energy pulse travels through the water, reflects off an object and returns to a receivers. Typically the object is not uniform in shape so energy may reflect along a number of different axes from the objects. Moreover, each of these reflections may travel back to the receiver through diverse paths caused by reflections from different thermal layers in the waters the water surface or the sea bottom. Consequently the reflected signal is corrupted with phase discontinuities caused by the diverse arrival times of these reflections along paths of different length. Moreover, it is often made more difficult to detect the signal because the reflected signal is further burdened with noise accumulated as the signal propagates through the medium.
As known, various approaches have been used to recover the true or coherent transmitted signal. For example, a single quadrature receiver has been used to integrate a received signal over an integration interval corresponding to its pulse width with some success. However, it has been demonstrated that a priori knowledge of phase discontinuities in the received signal can significantly enhance the probability of signal detection by such a detection modality. That is, knowledge of the phase discontinuities would enable a parallel quadrature receiver or other signal processing circuit to optimize its operation over coherent intervals within the received signal structure. Processing in such receivers over such coherency intervals would enhance the probability of detection over the probabilities achieved with a single quadrature receiver.
Neural networks have also been used to detect phase by detecting persistent patterns of phase in a time varying or oscillatory signals. U.S. Pat. No. 5,146,541 (1992) to Speidel discloses a signal phase pattern sensitive neural network system that employs duplicate inputs from each of its sensors to the processing elements of a first layer of a neural network. One input is always phase shifted relative to the other. This system employs a modification of a conventional Kohonen competitive learning rule that is applied by the processing and learning elements of a second layer of the neural network. Partitioned segments are then processed and a third layer of the neural network that comprises processing elements connected to the second layer processes the different segments to identify a desired characteristic of the incoming signal.
Thus the prior art discloses a need for defining coherency intervals in a received signal characterized by phase discontinuities in the signal and by background noise. The prior art also discloses the use of neural networks for discerning patterns in phase. However, there remains a need for an apparatus and a method for detecting signal phase discontinuities that may occur at random times, determining the presence and duration of coherency intervals within the received signals and utilizing these coherency intervals to enhance detection capabilities.